The present invention pertains generally to Humidity Sensors and more particularly to sensors and methods of fabricating sensors for detecting relative humidity and temperature.
Conventional humidity sensors utilize an aluminum layer which is partially anodized on its upper surface prior to deposition of the top electrode. The upper electrode is sufficiently thin to allow passage of water molecules to the Al.sub.2 O.sub.3 layer which changes resistance and capacitance when employed in a conventional ac stabilization circuit.
Although conventional humidity sensors have been developed to provide good response, slow anodization of the aluminum base material causes progressive impedance and capacitance change in the sensor due to exposure to moisture and residual acid remaining in the Al.sub.2 O.sub.3 layer after the initial anodization process, especially when subjected to an ac current as utilized in conventional humidity sensor circuitry. As a result, the humidity detecting circuitry must be continually adjusted for proper operation throughout the life of the device. Even more seriously, use of the device in high temperature environments causes rapid aging even though appreciable amounts of moisture may not be present.
Attempts to overcome these problems by methods such as pre-aging the sensor, as disclosed in the prior art, have had limited success due to the non-transient nature of the slow anodization process.
Another attempt to overcome these problems is disclosed in U.S. Pat. No. 4,143,177 issued Mar. 6, 1979 to Kovac et al wherein a substantial portion of the Al metal present in the Al layer is removed by anodizing the Al layer using conventional anodization methods. As set forth, this process is used in an attempt to provide high temperature stability of the absolute humidity sensor disclosed by Kovac et al since a major portion of the Al metal left in the sensor which can become oxidized during operation or storage of high temperatures, is removed, which would otherwise affect the sensitivity and, consequently, calibration of the sensor.
However, since the Al.sub.2 O.sub.3 layer of the Kovac et al sensor is formed according to conventional anodizing methods, the Kovac et al humidity detector is incapable of providing a linear response with respect to relative humidity. Rather, the Kovac et al detector can only provide a non-linear response to absolute humidity due to the structure and density of the Al.sub.2 O.sub.3 layer resulting from the manner in which the Al.sub.2 O.sub.3 is produced, i.e., with conventional anodizing methods. Furthermore, conventional methods of anodizing utilized by Kovac et al cannot remove essentially all the Al metal and other impurities in the Al.sub.2 O.sub.3 layer, but only a substantial portion, as set forth in U.S. Pat. No. 4,143,177.